Sunshine nudges asteroid into odd orbit

New NASA measurements of the orbit of the half-kilometer asteroid 1999 RQ36 have given space science its most precise measurement of such space rocks' orbit – and revealed a 160km deviation from the orbit predicted by gravity.

The drift, which showed up by comparing observations made in 1999, 2005, and September 2011, is due to the asteroid absorbing sunlight which it then emits as heat, known as the Yarkovsky effect (after a Russian engineer that first proposed the phenomenon in the 19th century).

Although Standard Register units of London Bus and swimming pool aren’t available for this story, the scientists provide some handy yardsticks: at its closest pass to the Sun, 1999 RQ36 experiences an extra push of about “half an ounce, about the weight of three grapes on Earth”, according to Steven Chesley of the JPL. “Meanwhile, the mass of the asteroid is estimated to be about 68 million tons.”

To measure the asteroid’s drift, astronomer Michael Nolan’s team measured the object’s distance to the Arecibo Observatory to an accuracy of 300 meters, “like measuring the distance between New York City and Los Angeles to an accuracy of two inches”.

These measurements were then used by NASA to determine that in the period 1654 to 2135, 1999 RQ36 will rack up 11 passes of Earth closer than 7.5 million kilometers.

The calculations will also form an important input in planning the OSIRIS-REx mission, which is due to launch in 2016 and rendezvous with the asteroid in 2019, grabbing a 60 gram sample to bring back in 2023. ®